1. Field of the Invention
The present invention generally relates to liquid crystal display devices, and more particularly to a liquid crystal display apparatus integrated with a driver circuit formed on a glass substrate.
A liquid crystal display device is compact, light and low power consumption, as compared to a display device with a CRT (Cathode-Ray Tube), and is widely used as a display device of a portable computer or the like. Generally, the liquid crystal display device has a structure in which two transparent substrates sandwich liquid crystal. Opposing electrodes, a color filter and an alignment film are provided on one of two opposed surfaces of the respective transparent substrates, and thin-film transistors (TFTs), pixel electrodes and an alignment film are provided on the other opposed surface. Polarization plates are respectively provided to the surfaces of the transparent substrates opposite to the respective opposed surfaces. The two polarization plates are arranged so that the opposed axes thereof are orthogonal to each other. In this arrangement, light is allowed to pass through the polarization plates without an electric field applied, and is shielded with an electric field applied. This is called normally-white mode. When the polarization axes of the two polarization plates are parallel to each other, a normally-black mode is obtained. Hereinafter, the transparent substrate with the TFTs and the pixel electrodes formed thereon may be referred to as a TFT substrate, and the other transparent substrate with the opposed electrodes formed thereon may be referred to as an opposed substrate.
2. Description of the Related Art
Recently, a polysilicon TFT has been attractive because a liquid crystal display part and a peripheral circuit part can be integrally formed. The electron field effect mobility of a polysilicon TFT is approximately equal to tens of cm2/Vs to 200 cm2/Vs is thus 1/10-¼ of that of a single-crystal silicon MOSFET. Hence, it is difficult to form a high-speed circuit which operates at tens of MHz by using polysilicon TFTs in the liquid crystal display device. Further, it is also difficult to form a complex circuit in the liquid crystal display device using polysilicon TFTs due to a limitation on a relative large design rule (generally 3-5 μm) applied to a glass substrate used in the liquid crystal display device.
For the above reasons, the conventional liquid crystal display device using the polysilicon TFTs employs a divided dot-sequential drive method in order to display an image on a display part. A control circuit is provided outside of the display part and is used to divide display data from a data driver into parts in order to reduce the frequency of the display data. This is because the data driver formed of polysilicon TFTs do not operate at tens of MHz. The display data is written into data signal lines to which analog switches are connected, and are then supplied to polysilicon TFTs which are on via the analog switches which are also on. Hence, the liquid crystal layers on the pixel electrodes are operated so that an image can be displayed.
Also, the conventional liquid crystal display device has another disadvantage in that the analog switches are required to have a comparatively wide channel width in order to complete write data into the pixels for a short time. Thus, it is required to provide a large area on the glass substrate for forming the analog switches.
Further, the conventional liquid crystal display device uses the control circuit provided outside thereof in order to divide the display data into parts to thus reduce the frequency of the display signal. Hence, it is required to divide each of the R, G and B signals which are respectively a one-channel signal into a plurality of channels based on the number of divisions. For example, if the display data is divided into 16 parts, each of the R, G and B signals is divided into 16 parts, so that the display data is divided into 48 channels in total. Furthermore, the liquid crystal display device using the polysilicon transistors is required to have the function of converting the display signal in digital formation into an analog signal which actually drives the liquid crystal display part and to thus have a specific IC chip for controlling the polysilicon TFTs. This increases the cost. Moreover, the control circuit provided outside of the display part consumes a certain amount of power and is not suitable for a digitized interface.
The polysilicon TFT can be formed by a low-temperature process (lower than a process temperature of 600° C.). When such a polysilicon TFT thus produced is applied to the liquid crystal display device, a display failure may occur. Examples of a display failure is a scan stripe, a warp streak, a ghost display and an unevenness between horizontal display and vertical display. The display failure results from a periodic performance change of the low-temperature polysilicon TFTs, deviations of the performance of the analog switch TFTs and delays of time of signals caused in a shift register and a buffer circuit, which circuits form the data driver.
The periodic performance change of the low-temperature polysilicon TFTs results from a factor of instability of an eximer laser oscillator. An energy error ΔE (=Emax−Emin) always exists between pulses of the eximer laser, and is greater than 10% of Emax if the frequency of the laser pulse falls within the range of 50 to 300 Hz where Emax denotes the maximum energy value of the eximer layer and Emin denotes the minimum energy value thereof. On the other hand, the range of the projection energy within which the crystallization of the polysilicon TFTs can be ensured is approximately equal to ±3-5% of an optimal projection energy Eop. As described above, sine the maximum and minimum energy values Emax and Emin of the eximer laser is located outside of the projection energy range of the laser pulse within which the crystallization of the polysilicon transistors is ensured. Hence, the low-temperature polysilicon TFTs have a dispersion of the performance.
There is also a dispersion of the crystallization of the low-temperature polysilicon TFTs. This is because the crystallized state of polysilicon is changed at an interface portion in which the laser beams overlaps each other when scanning the glass substrate. Hence, the performance of the polysilicon TFTs, such as the electron field effect mobility or the threshold voltage thereof will be changed.
The delays of the signals caused in the shift register of the driver circuit result from an arrangement in which the data driver operates at a high frequency in the divided dot-sequential drive method and the shift register has a large number of stages.